1. Field of the Invention
The present invention relates to systems that can be used to carry out underwater seismic prospecting or exploration, especially when the seafloor is at a great depth. The system of the invention can be used especially to detect the presence of petroleum in ocean beds.
2. Description of the Prior Art
There are known ways of seismic prospecting for petroleum on the seafloor or seafloor using techniques derived from those used in onshore seismic prospecting for petroleum. The differences relate essentially to the excitation means used to obtain the necessary acoustic waves as well as the means used to position the acoustic sensors in the face of the difficulties caused by the great depth of water which, unlike in onshore prospecting, does not allow for the manual positioning of these elements at the required positions.
It becomes necessary in this case to fixedly attach the acoustic sensors, which are geophones or hydrophones for example, to very long cables that are towed behind a boat and submerged at suitable places to make the necessary measurements. In practice, cables having lengths of several kilometers are used. They are placed parallel to each other with a specified distance between them.
At present, for relatively shallow seafloors, with depths of about 200 meters, these cables are submerged by being reeled out behind the towing boat. When the measurements have been completed at one place, they are raised to the surface and moved further away. The operations of raising and sinking the cables entail idle times between the series of measurements. These idle times are prohibitively costly for depths of over 200 m. Furthermore, there is a series of technical problems, already present at the shallow depths, that becomes excessive at great depths. Since the precision required for positioning the sensors, and hence the cables, on the seafloor in relation to the theoretical grid of values stipulated for accurate measurements is in the range of xc2x120 m for submersion depths of up to 3000 m, it is easy to imagine the difficulty of obtaining such precision, given the drifts that the cable undergoes when it sinks towards the seafloor.
It is also necessary to obtain a renewable coupling of the geophones with the seafloor, whatever its nature and relief. The greater the depth, the more limited is the knowledge of these parameters and the more difficult is it difficult to handle the cable in order to adapt to it.
The rise in productivity as well as in the quality of the images restituted from the measurements are leading to the parallel use of several cables to obtain 3D images. In this case, the relative positioning of these cables increases the constraints on the individual positioning of each of the cables.
As already stated above, it soon becomes necessary to be able to rapidly and safely shift the cable between two series of measurements without having to raise and then sink them again.
To enable a maneuver of this kind, the invention proposes a system of undersea seismic prospecting especially for seafloors of the type comprising at least one measurement streamer provided at its head with a diving vehicle and, at its tail, with an inert float wherein chiefly the streamer is formed by a non-stretching and non-compressible inner tube and an outer tube that is coaxial with the inner tube and radially extensible, wherein the system comprises means to inject a liquid into the space between these two tubes so as to enable the setting of the buoyancy of the streamer between a negative value at which it lies on the seafloor and a positive value at which it floats above the seafloor so that it can be shifted one measurement position to another.
According to another characteristic, the inert tail float communicates with the streamer so that it receives a part of the liquid injected into it in order to modify its buoyancy in the same way as that of the streamer.
According to another characteristic, the interior of the tail float is divided into two compartments separated by a floating piston, a first compartment into which the liquid coming from the streamer penetrates and a second compartment linked to the depths of the sea to ensure the balance of the pressures.
According to another characteristic, the diver vehicle comprises a container of liquid and means to inject the liquid into the streamer and retrieve it.
According to another characteristic, the interior of the diver vehicle is divided into two compartments separated by a floating piston, a first compartment comprising the liquid designed to be injected into the streamer and a second compartment linked to the depths of the sea to provide for the balance of the pressures.
According to another characteristic, the streamer and the tail float are provided with guide ropes used to maintain the altitude of the assembly at a specified value above the seafloor when their buoyancy has become positive following the injection of the liquid.
According to another characteristic, the vehicle comprises means of navigation to make it maneuver in a horizontal plane so as to adjust the offset of the streamer with respect to the towing axis of the towing boat of the assembly.
According to another characteristic, these means of navigation comprise at least one propeller located in a cross-tunnel opening out on each of the lateral flanks of the diver vehicle.
According to another characteristic, the means of navigation comprise a set of streamers designed to be towed in parallel and measurement means to determine the spacing between these streamers and control the navigation means of the diver vehicle accordingly.
According to another characteristic, the diver vehicle is provided with a keel enabling it to get firmly anchored to the seafloor.
According to another characteristic, the modules are high-density modules and enable strong mechanical coupling with the seafloor.